The invention relates to a method for producing flexographic printing plates and also to apparatus suitable therefor.
The most widespread method for producing flexographic printing plates involves the imagewise exposure of the photopolymerizable, relief-forming layer with actinic radiation, especially longwave UV radiation, through a mask produced digitally or photographically. In a further method step, the exposed layer is treated using a suitable solvent or solvent mixture, with the unexposed, unpolymerized regions of the relief-forming layer being dissolved, while the exposed, polymerized regions are retained and form the relief of the printing plate.
Digital imaging of photosensitive flexographic printing elements is known in principle. In this context, flexographic printing elements are not produced conventionally, by placement of a photographic mask, followed by exposure through the photographic mask. Instead, the mask is produced in situ directly on the flexographic printing element by means of appropriate technologies. Flexographic printing elements may be provided, for example, with opaque, IR-ablative layers (EP-B 654 150, EP-A 1 069 475) which can be ablated imagewise by means of IR lasers. Other known technologies include layers which can be written by means of inkjet technology (EP-A 1 072 953), or layers which can be written thermographically (EP-A 1 070 989). Following the imagewise writing of these layers by means of the technologies appropriate for the purpose, the photopolymerizable layer is exposed through the resultant mask by means of actinic light.
Imagewise exposure with actinic radiation takes place on a standard basis using UV radiation sources which possess significant emission in the range from about 315 nm to 420 nm (longwave UV region to violet region of the visible spectrum). The most frequently used radiation source are UV/A tubes, which possess an emission maximum at a wavelength of about 370 nm and generate UV intensities of 10 mW/cm2-30 mW/cm2, measured at a distance of 50 mm (typical distance from the radiation source to the surface of the flexographic printing element). UV/A tubes of this kind are available for example under the “R-UVA TL 10R” designation from Philips.
Use is also made, moreover, of mercury vapor lamps for the imagewise exposure, with preference being given to doped medium-pressure mercury vapor lamps, since doping with iron and/or gallium allows an increase in the fraction emitted in the UV/A region. Units which comprise at least one mercury vapor lamp and also a reflector are referred to below as UV lamps. As well as the stated fractions of UV/A radiation, the emission spectrum of UV lamps also includes fractions of UV/B and UV/C radiation. In the course of imagewise exposure, these shorter-wave UV radiation fractions may lead to unwanted side-effects, such as, for example, to embrittlement of the irradiated surface or to the formation of ozone. Usually, therefore, medium-pressure mercury vapor lamps are selected for the imagewise exposure, where appropriate selection of the lamp glass greatly reduces the emission of UV/B and UV/C radiation. Furthermore, filters are also employed that substantially absorb the UV/B and UV/C radiation fractions, yet are substantially transparent to UV/A radiation. Since the majority of UV lamps available convert about 40% of the consumed electrical power into thermal radiation, the intensity of the UV lamps, which is actually high, cannot simply be transferred directly onto the substrate, since an excessive temperature load may damage the flexographic printing element. In order to reduce the thermal load on the substrate that is to be irradiated, the distance selected for the UV lamp to the substrate must be relatively large, 500 mm, for example, thereby reducing the UV intensity impinging on the substrate. By means of special reflectors and/or mirrors which are at least partly transparent to infrared radiation, but substantially reflect UV radiation, it is likewise possible to reduce the temperature load on the substrate that is to be irradiated. Nevertheless, it is usually not possible to realize UV/A intensities of >100 mW/cm2 at the level of the flexographic printing element, since the element, as a result of the severe heating, will otherwise be damaged and additionally, in the case of flexographic printing elements with a PET film substrate, there is a risk of a loss of dimensional stability.
However, for the radiation curing of photopolymerizable compositions, there is also increasing use of LEDs (light emitting diodes) which emit UV light.
Common LED systems for UV curing are focused presently in practice on the wavelengths 395 nm and also 365 nm. Other possible spectral ranges are 350 nm, 375 nm, 385 nm, and 405 nm. Scientific publications additionally mention the wavelengths 210, 250 nm, 275 nm, and 290 nm. LEDs are distinguished by a narrow intensity distribution (typically +/−10-20 nm). They have no significant warm-up phase and can be regulated to about 10% to 100% of the maximum intensity.
Using UV light-emitting diodes it is possible to achieve power levels of a few watts/cm2, and the efficiency, depending on LED-UV system, is between 1% and 20%. The rough rule of thumb here is as follows: the shorter the wavelength, the lower the efficiency. And the shorter the intended emission wavelength, the higher the production costs.
At the present time, LED systems for areal curing are available commercially with a wavelength of 395 nm and a UV power between 1-4 W/cm2, and with a wavelength of 365 nm in the 0.5-2 W/cm2 range, from various suppliers.
In order to allow quicker curing rates, the suppliers of LED units are currently boosting the UV output power at the expense of the efficiency. The currently most powerful LED units have an efficiency of around 8-12% at 395 nm, whereas a medium-pressure mercury lamp is located at 28% efficiency. The efficiency of a 365 nm LED unit is currently below 10%.
LED arrays are very expensive. The current price for an 8×1 cm UV-LED array is 5000-6000 euros. If the web width is doubled, then for an LED assembly there is also a doubling in the number of LEDS and hence also in the price. In the case of mercury vapor lamps, the price difference between different lamp lengths is smaller.
U.S. Pat. No. 6,683,421 discloses a device for the photocrosslinking of photoreactive materials, comprising (a) a housing, (b) a light-emitting semiconductor array mounted to the housing and capable of emitting light with a wavelength suitable for initiating photoreactions, (c) a power source for energizing the array to emit light, (d) a control unit coupled to the power source, for regulating the power supplied by the power source to the array, wherein (e) the array consists of a plurality of light-emitting semiconductors, and (f) the plurality of semiconductors is organized in a plurality of groups. No specific use applications are given for the apparatus described.
U.S. Pat. No. 6,931,992 discloses a system for exposing a photopolymerizable element with UV light, comprising a rotation means for rotatively moving the photopolymer, and a radiation source assembly arranged around the rotation means, the assembly comprising at least one radiation source which is able to deliver at least two different light emissions onto the photopolymer and which can be moved at least partly transverse to the direction of rotation and along the photopolymer, the different light emissions being arranged such that their rays overlap one another, in order to allow exposure of all points on the surface of the photopolymer continuously with at least one radiation source. Also described is a system for ablating a flexographic printing plate and carrying out exposure with UV light. Specific radiation sources identified are mercury plasma capillary lamps.
WO 2008/135865 describes a method comprising the positioning of a printing plate with photocrosslinkable material on an imaging unit, the imaging of the plate in accordance with image data, the application of UV radiation from a plurality of UV-emitting diodes for crosslinking the photocrosslinkable material on the plate during the imaging of the printing plate, where the printing plate may be a photopolymerizable flexographic printing plate, a photopolymerizable letter press printing plate, or a photopolymerizable sleeve. Additionally described is the removal of the plate from the imaging unit and its subsequent exposure from the reverse or from the front and optionally also from the reverse, with UV radiation from a plurality of UV-emitting diodes.
DE 20 2004 017 044 U1 discloses apparatus for exposing screen printing stencils, offset printing plates, flexographic printing plates or the like, having at least one light (1), having a transparent bearing plate (8) for an item intended for exposure, and having a means (10, 11) for moving the at least one light (1) backward and forward, the at least one light (1) being arranged at a small distance from the bearing plate (8), characterized in that the light (1) has at least one UV light-emitting diode (3).
During the exposure of photopolymer plates with UV light through a mask produced by laser ablation, an unwanted effect which occurs is the inhibition of the polymerization as a result of oxygen, which diffuses into the photopolymer layer from the surrounding atmosphere. The same effect occurs if a layer imagable digitally by means of other technologies is employed, since these layers are generally only a few micrometers thick and hence are sufficiently thin that the oxygen from the ambient air is able to diffuse through them.
When exposing the flexographic printing element through a photographic mask it is necessary to ensure that the negative is lying uniformly on the surface of the flexographic printing element, without air inclusions, since otherwise there may be instances of faulty exposure (“hollow copies”). On the photopolymerizable layer there is therefore usually a substrate layer which is less tacky than the surface of the photopolymerizable layer; on the other hand, it is usual to use film negatives having at least one rough film side. Lastly, by the application of reduced pressure (with the aid of a vacuum film, for instance), intimate contact between film negative and plate surface is produced, with the air present between them being very largely removed. Consequently the oxygen is no longer able to inhibit the photopolymerization. The most frequently used UV beam sources, namely UV/A tubes, possess a very diffuse light. Scattered light plays a significant part, promoted with a low UV intensity and the associated long exposure time. The UV/A light is scattered at the vacuum film and at all of the boundary faces (e.g., between film negative and plate surface). As a result, there may easily be a widening of positive elements that are to be imaged, while fine nonimage-region structures may be reduced in size.
Inhibition of polymerization by oxygen may also lead to severe element reduction, since, at the edges at least, the image elements no longer undergo sufficient polymerization and are ultimately removed by solvent, for example, in the course of the imagewise exposure. The result of this is what is called a reduction in tonal value—that is, the tonal value measured on the printing plate for a screen of positive elements (halftone dots) is smaller than the value corresponding to the image data. In certain circumstances this may be desirable, in order, for example, to compensate the increase in tonal value in the printing operation itself; on the other hand, below a certain tonal value, screen dots are no longer stably anchored and will no longer be imaged. As a result, gray gradations are lost and the tonal value range in the print is lower. The effect of tonal value reduction during the exposure of digital photographic printing plates is known according to the prior art with UV/A tubes. As a result of the polymerization-inhibiting effect of the oxygen during exposure, the polymerization of the halftone dots is disrupted, and so the halftone dots on the plate will be smaller than provided for in the data.
From the photographs it can be seen that the roughness of the substrate surface after the method of the invention has been performed can be reproduced on the printing surface, whereas in the case of exposure by means of conventional UV/A tubes, the inhibitory effect of the oxygen means that the roughness is not transferred to the surface of the printing element.